Carbon catalysts play an important role in various chemical processes from industrial to pharmaceutical settings. Carbon catalysts enable chemical reactions to occur much faster, or at lower temperatures, because of changes that they induce in the reactants. Carbon catalysts may lower the energy of the transition state of chemical reactions, thus lowering the activation energy. Therefore, molecules that would not have had the energy to react, or that have such low energies that it is likely that they would take a long time to do so, are able to react in the presence of a carbon catalyst by reducing the energy required for the reaction to occur. Not only do carbon catalysts increase the rate of reaction, but they may also drive a reaction towards the desired product.
Typically, catalysts are applied to a substrate before introduction to a chemical process. Desirably, the substrate holds the catalyst while presenting the catalyst to reactants in the chemical process. Conventional catalyst substrates, or supports, include carbon or ceramic granules arranged in a bed, and ceramic monoliths.
Traditionally, carbons utilized as catalyst supports are either granules or powders. In a perfect world, carbons used for catalyst supports would be chosen only for their activity and selectivity. The more common features that are important factors in determining activity and selectivity are surface area, pore volume, pore size, ash content, friability, availability, and/or other elements contained in the carbon matrix. The foregoing are not the only desirable features; rather, they are ones that are known to be obtainable within the art.
Conventionally, carbon catalyst supports are chosen more for properties that meet parameters of the chemical process, than for features that would make purely the best catalyst, for highest activity and selectivity. While a particular carbon substrate might have the best features for activity and selectivity, it may not be the best choice considering the chemical process parameters. For example, carbon granules suffer from attrition making exact pressure drop determinations difficult, and they scale up poorly in chemical processes. When chemical reactants trickle through a bed of granular carbon catalyst, the catalyst must be as attrition resistant as possible, less the bed collapse and flow cease or the catalyst metals be lost. Attrition is a particularly aggravating issue, because it alters the physical parameters of the chemical process as it proceeds, and causes financial loss, particularly when the catalyst is a precious metal. For this reason, carbons of choice are typically nutshell carbons, which are durable, but which have very small pores that can harshly limit activity and selectivity. When a powder carbon catalyst is stirred violently in a batch reactor with chemical reactants, the carbon catalyst must be non-friable to some degree to allow it to be economically separated from the reaction at termination in order to prevent loss of the catalyst. Thus, perhaps one must exclude carbons with better catalytic properties, but which are too friable.
Ceramic catalytic monoliths have been used in the art for advantages they provide over fixed bed supports, such as predictable pressure drop through the catalyst bed, scalability based on a model that predicts performance through incremental increases in volume of catalyst with respect to the same reactant volume flow, separation of the catalysts from the reaction and from the product stream, practical continuous operation and ease of replacement of the catalyst, and layering of the catalyst or the catalysts either on the monoliths' wall depth or wall length, or both. The low pressure drop of catalytic monoliths' allows them to operate at higher gas and liquid velocities. These higher velocities of gas and liquids promote high mass transfer and mixing.
Catalytic monolith development has been an ongoing process in an effort to enhance catalytic activity, catalytic selectivity, and catalyst life. Although monoliths have advantages over fixed bed supports, there are still problems associated with traditional ceramic monoliths. Exposure of the catalytic metal in the catalytic monolith to the reactants is necessary to achieve good reaction rates, but efforts to enhance exposure of the catalytic metal often have been at odds with efforts to enhance adhesion of the metal to the monolith substrate. Thus, catalytic ceramic monoliths have fallen short of providing optimal catalytic selectivity and activity.
As seen below, ceramic carbon catalyst monoliths developed to date, on one hand may provide good selectivity and activity, but on the other hand may not be suitable for process parameters such as durability and inertness. Conversely, ceramic carbon catalyst monoliths suitable for such process parameters may have diminished selectivity and activity. Thus, it would be ideal to take a carbon with the best features for a catalyst based on its activity and selectivity, and then form a carbon monolith catalyst to fit the process parameters of choice.
There have been efforts to form a carbon support that would have some of the features of a ceramic monolith catalyst. These efforts fall into three general classes: gluing or binding of carbon granules or powder to form larger structures, coating ceramic monoliths with an organic compound such as sugars or liquid polymer plastics, followed by carbonization of the organic compound on the ceramic monolith, and formation of a structure from an organic material, such as a plastic or nylon, followed by carbonization of the structure.
The binding of carbons gives some degree of choice of carbon precursor, but the result is a carbon support with the binder as a new element. These binders can vary from organic glues to pitches. In most cases, the binders are susceptible to attack by the reaction media in application. Some cause side reactions, or poison the catalyst. Furthermore, the result is a random binding of granules, or the creation of a new granule—a chopped extrudate of powdered carbon and binder. In either case, the parameters of flow are not predictable by simple, understandable models. Although the carbons selected have generally been in use as unbound catalyst supports, and unbound activity and selectivity information on the carbon can sometimes be used, still the binder is not inert, and therefore binder influence is always an issue.
Carbonization of an organic material forms a support with little hope of prior carbon activity or selectivity information. Because the carbon is formed each time the support is prepared, and is limited to those precursor and organic materials that can be coated or formed and carbonized, commercially available carbons, known in the art to produce excellent catalyst, are excluded from consideration. Furthermore, the carbons normally used in preparation of catalyst supports are prepared from naturally occurring materials such as wood, peat, nutshell, and coal, and not from refined or organic chemicals. Carbon produced from naturally occurring material is known to retain some of the beneficial structural characteristics as well chemical nature of the precursor material. These characteristics are known to be important to the final activity and selectivity of the catalyst. While carbonization may be a way of producing a carbon coating or structure, it extends marginally the catalyst art, and does not produce a catalyst utilizing the known carbon methods of choice in the art.
Thus, there is a need in the art for a carbon monolith catalyst, and a process for making the same, having attrition resistance, predictable pressure drop, high selectivity, high activity, and scalability for commercial economy and efficiency. More particularly, there is a need in the art to provide an activated carbon monolith catalyst which allows the manufacture of the catalyst of choice to fit the process parameters, while increasing the utility of the catalyst with predictable pressure drop through the catalyst bed, scalability based on a model that predicts performance through incremental increases in volume of catalyst with respect to the same reactant volume flow, separation of the catalysts from the reaction and from the product stream, practical continuous operation and ease of replacement of the catalyst, and layering of the catalyst or the catalysts either on the monolith's wall depth or wall length, or both, and while providing high selectivity and activity.